This invention relates to a method for the manufacture of a super-fine homogeneous powder composition for producing cordierite having components in desired component ratios.
Cordierite (2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2) exhibits very low thermal expansion over a wide range of temperatures and offers outstanding resistance to the thermal shocks exerted thereupon in case of an abrupt temperature change. Cordierite, therefore, is widely used in refractory heat plates, honeycomb structures for cleaning exhaust gas from automobiles, and physicochemical refractory materials.
Generlly cordierite is obtained by combining kaoline, talc, magnesite, aluminum oxide, magnesium hydroxide, clay, etc. and burning the resultant mixture. Theoretically, cordierite is composed of 13.7% of MgO, 34.9% of Al.sub.2 O.sub.3, and 51.4% of SiO.sub.2. Generally, the MgO content of cordierite may vary within the range of 10 to 16%, the Al.sub.2 O.sub.3 content within the range of 33 to 41%, and the SiO.sub.2 content within the range of 43 to 56%. Recently, synthesis of cordierite by use of synthetic raw materials instead of naturally occurring raw materials has come to prevail. It has been ascertained that presence of impurities, particularly calcium oxide and alkalis, increases the thermal expansion coefficient and exerts adverse effects upon the ability to resist thermal shocks.
With a view to producing cordierite of high purity, there have been proposed melting methods. Methods of this kind developed to date have invariably proved impracticable. Various methods developed for the production of the composition 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2 by the combination of MgO, Al.sub.2 O.sub.3, and SiO.sub.2 each of high purity generally comprise combining raw materials as shown below and causing reaction of the raw materials by burning the resultant mixtures.
(1) [2MgO+2Al.sub.2 O.sub.3 ] (spinel) and 5SiO.sub.2 PA0 (2) Reaction product of 2MgO+5SiO.sub.2 and 2Al.sub.2 O.sub.3 PA0 (3) Reaction product of 2Al.sub.2 O.sub.3 +5SiO.sub.2 and 2MgO PA0 (4) 2MgO+2Al.sub.2 O.sub.3 +5SiO.sub.2 (In this case, the oxides are mixed and are allowed to react in the mixed state.)
These are all powder mixing methods. Particularly in the case of (1), the reaction involved requires a mineralizer (such as, for example, LiF or MgF.sub.2) and entails a possibility of causing air pollution with fluorine. This method has a disadvantage that the product is deficient in electrical properties and refractoriness.
When the raw materials of high purity are mixed in amounts conforming to the theoretical composition of cordierite and the resultant powder mixture is pressed into a desired shape, set in a furnace, and fired, the product of the firing exhibits the lowest thermal expansion coefficient and high resistance to thermal shock. By this firing method, since the formation of liquid phase generally observed at low temperatures fails to occur, the firing temperature rises and, consequently, the temperature range in which the firing is allowed to proceed is narrowed, making difficult the firing itself. Moreover, the product of the firing exhibits properties which are not always consistent.
An object of this invention is to provide a novel method for the manufacture of a powder composition for producing cordierite which is readily converted into desired cordierite by firing.